Interconnect has largely been considered a passive element in any system, providing sufficient but non-ideal connectivity between different parts of the system. In that manner, a prior art twisted wire pair, whose cross-section is illustrated in FIG. 1, provides good connectivity for signals flowing in the wires, but is prone to energy loss that is proportional to the data rate, or the frequency of the transmitted signals. Energy loss in twisted wire pairs takes two principal forms, series resistance losses due to the finite conductance of the wires as well as skin-effect, and parallel energy losses due to the insulation dielectric that separates the two wires of a wire pair from each other. Whereas skin-effect loss (the primary series loss component) increases as the square-root of the operating frequency, dielectric losses are directly proportional to the frequency. Both contribute to substantial signal attenuation at high data rates.
Additionally, electromagnetic coupling between wires, both near to, and at a distance from a signal wire contributes to distorting the signal conducted by the wire. Such undesirable coupling of signal energy, called ‘crosstalk’, takes two principal forms, capacitive and inductive. Capacitive coupling as the term indicates occurs due to the finite capacitance present between a signal wire and a coupling neighboring wire. Inductive coupling occurs due to the magnetic fields created by currents flowing in neighboring or distant wires that creates corresponding electro-motive force in the wire carrying the signal of interest. Both coupling phenomena lead to the addition of noise into a signal, degrading signal integrity and thereby increasing the probability of erroneous registration of the signal in a receiver system. Means of minimizing this degradation are therefore of much importance to communications systems employing wires to transmit signals.
The prior art twisted wire pair as well as standardized cables such as Cat-5e, Cat-6 (different categories) addresses such concerns of electromagnetic coupling. A wire pair consists of two individual wires coupled strongly and placed close to each other providing a means for ‘differential signaling’, a technique whereby a signal and its complement are transmitted simultaneously and the corresponding symbol recognized as the difference between the two electrical quantities received. Differential signaling largely eliminates concerns with any differences in ground or reference potentials between the communicating systems. Additionally, differential signaling makes it possible to employ high-gain amplifiers to recover an attenuated signal as long as the polarity relationship between individual signals of the differential pair is maintained. Thus, for example, a 1V swing binary, differential signal, with an effective difference between the two wires of 0.5V, may still be recognized correctly despite 10× attenuation down to 50 mV by a differential amplifier, provided that the polarity relationship between the true and complementary individual signals is maintained. Any distant-source noise that couples electro-magnetically into this wire pair couples in very much the same manner into both wires, thereby retaining the difference signal the same, and causing no significant degradation in signal integrity as long as the receiver differential amplifier is capable of rejecting this ‘common-mode’ noise. But a wire pair lying adjacent to another wire pair may not see such a benefit, such as in a flat-tape cable where signal wires as arranged in a bonded fashion adjacent to each other. This problem is effectively addressed by twisting the wires of the wire pair around each other. Over a sufficient length, because of the twist, the coupled noise from any adjacent signal wire sums out to be the same on both individual wires of a twisted wire pair, thus again rendering such noise ‘common-mode’. As an additional enhancement, standard cables such as Cat-5e also offset the twists of wire pairs with respect to each other, starting with a low twist rate for one wire pair and tightening the twist rate for other included wire pairs in the cable assembly.
Twisted wire pairs also cancel out electromagnetic emissions from the signal wires, diminishing electromagnetic interference (EMI) with other systems. Perhaps the very first instance of such a brilliant application of this prior art is the twisting of the wires providing alternating current electricity to lamps and other electrical systems in buildings, minimizing the noise heard in entertainment radio devices. Additionally, twisted wires remain physically close, albeit somewhat inadequately, as a consequence of the intertwining of the wires, thus maintaining relative uniformity in their impedance and good coupling to each other.
Due to the reasons discussed, twisted wire pairs are very commonly employed for electrical signaling within electronic system boxes as well as between these boxes, such as between computers, and from video content players and high-definition displays. But as the volume of data exchanged continues to grow, some of the deficiencies of twisted wire pairs manifest themselves as limitations. A key such limitation is intra-pair skew, or the inequality in the total effective length of one wire with respect to the other in a wire pair. This asymmetry arises because of the independent manner in which the two wires are tensed and twisted with each other. The inequality typically increases with increasing length of the wire pair. In electrical terms, any such inequality in length gives rise to a delay difference between the traveling true and complement signal transitions in binary signaling, transforming part of the differential signal into a common-mode signal. For example, if the effective delay difference at the end of a long length of a wire pair is an inch, this will correspond to approximately 100 ps or more of delay difference at the end of the wire pair depending upon the insulator electrical characteristics. If a true and complement signal (a rising edge and a falling edge for voltage signals, for example) were to be launched simultaneously at the transmitter end on this wire pair, they would be offset at the receiver end of the wire pair by about 100 ps, potentially rendering the signals the same for 100 ps at the beginning of the symbol period and similarly for 100 ps at the end of the symbol period. In other words, 200 ps of the symbol information in certain symbol sequences is transformed from differential to common-mode, and if the receiver further requires at least 200 ps of differential signal for correct recognition with low error, the maximum bit-rate that may be transmitted on this wire pair, even with signals of high signal-to-noise ratio, would be approximately 1/(400 ps) or 2.5 Gbps. The duration of differential signal transformed to common-mode also leads to electromagnetic emissions from the wire pair. Intra-pair skew in twisted wire pairs is hence a severe limitation to link performance, as studies in the industry have indicated as well [Ref. 4].
Additionally, twisted wire pairs are also prone to impedance discontinuities that arise due to the physical separation of the wires of the wire pair that may arise due to assembly errors. As the frequency of data transmission through wire pairs increases, these impedance discontinuities become more significant and impact signal integrity. Attempts to correct such problems include very tight twisting as is done in improved cabling solutions in the industry [Ref. 5]. Such designs further increase effective electrical lengths of the twisted wire pairs, increasing inter-pair (between wire pairs) skew and thereby increasing synchronization challenges between signals flowing in wire pairs within a cable assembly. Inter-pair skew is a problem usually addressed by realignment circuits in receiver systems. Typical values of inter-pair skew in Cat-5e cables resulting from twist offset are more than 1 nS per 10 meters of length.
Twisted wire pairs also occupy about 4 times the physical volume of a single wire and lead to bulkier and relatively inflexible cable assemblies.
As the definition and quality of 2-D images and audio in multimedia transmission increases, there is a need for significantly higher data rates and correspondingly high frequencies of operation of such links as defined in the High Definition Multimedia Interface (HDMI) specification [1]. In view of the varied and significant limitations in prior art twisted wire pairs and cable assemblies, there is a need to improve upon wire pair construction and cable architecture for such links.